What natural processes can cause a sidereal day to be significantly longer than a sidereal year?

The classic example here is Venus, with a sidereal day of 245 Earth days and a sidereal year of 224.7 Earth days - clearly less than its sidereal day. I wrote an answer related to this on Astronomy Stack Exchange that explains Venus's slow rotation (and why it has retrograde rotation). The sequence of events, according to Alemi & Steveson (2006), is as follows:

1. A large body collides with Venus in a giant impact.
2. The resulting debris coalesces into a disk and then eventually a moon, which moves away because of tidal acceleration.
3. Venus is hit by another large body, which reverses its rotation.
4. The moon moves inwards and collides with Venus.

Now, this merely produced a sidereal day that is only slightly longer than a sidereal year. It seems quite possible that the second giant impact could have resulted in a different scenario. The motion of the moon falling towards the planet will accelerate the planet's rotation a bit. All the second impact has to do is change the planet's angular velocity just enough prior to the collision with the moon.

I wish I could give you more specific information about the exact necessary velocity and angle of the impacting bodies. The problem is that the proposed giant impacts on Venus haven't been studied in nearly as much depth as the collision between Earth and Theia that gave rise to the Moon, and it's a much more complicated problem to deal with. In terms of strict feasibility, however, the giant impact $\to$ moon formation $\to$ second giant impact $\to$ collision with moon scenario can almost certainly work.

posted about 8 years ago

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